Optical communications systems are used for economically transferring large volumes of data over long distances. Economies are improved if fiber link length is extended without repeaters. Single-mode lasers are commonly used in long-haul networks. As is well understood in the art, the output of a single-mode laser is composed of phase and amplitude noise that is present in the output optical spectrum, along with desired single spectral tone. The broadband amplitude noise is called Relative Intensity Noise (RIN) measured in dBc. The narrowband noise is usually measured in MHz full width of the spectrum where the optical power density is −3 dB, −10 dB, or −20 dB, relative to peak power. This is referred to as the laser line-width. This narrowband noise is often predominantly phase noise. The line-width of a distributed feedback (DFB) laser is generally 1-to-10 MHz.
The performance of optical transmission systems with long-haul optical spans is impacted by significant amounts of net optical chromatic dispersion. On such systems, the phase noise of the laser source is substantially converted to amplitude noise by the action of the dispersion, at a level that can significantly degrade performance. In order to achieve coherent long-haul optical transmission and reception, a narrow laser line-width is required.
Nonetheless, it is desirable to be able to use lasers with larger line-widths in order to obtain other desired properties, such as a wide range of tuning wavelengths, without the propagation degradation associated with excess laser line-width. It is known to use control loops to tune the laser frequency by adjusting bias current in order to mitigate phase noise. The frequency tuning responses of lasers are in general due to thermal and carrier density effects, which are both a function of the electrical bias current. However, these effects have different phase responses, so the complex sum of the two effects creates an amalgamated tuning response that severely degrades in the region of 1 MHz. Frequency tuning does not help amplitude noise, and often creates more amplitude variations, rather than reducing them.
It is known to use an end-line phase modulator to correct “chirp” and other bounded phase excursions. Such modulators have been made using the electro-optic effect of Lithium Niobate. However, phase modulators have a limited dynamic range that is generally exceeded by phase noise output by most lasers. Phase modulators are also expensive to add to an optical transmission system.
There are known advantages to be derived from compensating for phase and amplitude noise. The advantages include the prevention of multi-path interference (further described in applicant's U.S. Pat. No. 5,999,258), and compensation of certain non-linear distortion effects, such as four-wave mixing, which is greatly facilitated by eliminating rapid perturbations caused by line-width noise. A detailed discussion of nonlinear optical effects is provided by Agrawal, Govind P., “Nonlinear Fiber Optics”, 2nd, Ed., Academic Press, Inc., San Diego, Calif., 1995 (ISBN 0-12-045142-5).
Various systems have been proposed for compensating for amplitude in optical signals. These systems typically operate in the optical domain by filtering or canceling the noise using different interference techniques that are well known in the art. Unfortunately, the introduction of optical components reduces the signal to noise ratio of the optical signal.
U.S. Pat. No. 6,304,369 entitled METHOD AND APPARATUS FOR ELIMINATING NOISE IN ANALOG FIBER LINKS, which issued to Piehler on Oct. 16, 2001, describes a transmission system that uses an interference technique for canceling RIN. The RIN is cancelled by transmitting two copies of the modulated signal over two respective optical fiber links extending between a sender and a receiver; and recombining them at the receiver. This method requires that half the intensity of the output be transported over each link, and introduces optical components that further reduce the optical signal strength.
A similar technique for cancelling narrowband noise is provided by Helkey in U.S. Pat. No. 6,441,932, which issued on Aug. 27, 2002. Helkey's system does not require two optical paths over a substantial part of the link, however it does introduce an optical attenuator, which reduces signal strength.
U.S. Pat. No. 5,761,225, entitled OPTICAL FIBER AMPLIFIER ELED LIGHT SOURCE WITH A RELATIVE INTENSITY NOISE REDUCTION SYSTEM, which issued to Fidric et al. on Jun. 2, 1998, teaches a method for amplifying, and reducing RIN in an emission of an optical power source. Fidric et al.'s system employs optical filters and a fiber amplifier to provide feedback to the power source. In accordance with Fidric et al., all of the compensation for RIN is applied in the optical domain prior to the modulation of data.
Accordingly, a method and apparatus for effectively reducing line-width of an optical output signal emitted by a laser in an optical communications transmission system remains highly desirable.